Titan arum is a giant among plants, with one of the largest flowering structures and one of the foulest odours in the plant kingdom. Its flowering is rare and unpredictable, and always grabs the headlines.

Species Profile

Geography and distribution

Amorphophallus titanum is restricted to Sumatra in the Indonesian archipelago. It is found growing in the rainforests of western Sumatra, on steep hillsides, at 120 to 365 m above sea level.

Description

Flowers: Titan arum has a massive inflorescence (flowering structure) consisting of a spathe (collar-like structure) wrapped around a spadix (flower-bearing spike). The spathe is the shape of an upturned bell. It is green speckled with cream on the outside, and rich crimson on the inside. It has ribbed sides and a frilled edge, and can be up to three metres in circumference. The flowers are carried on the lower end of the greyish-yellow spadix. At the base of the spadix, within the protective chamber formed by the spathe, is a band of cream male flowers above a ring of the larger pink female flowers. When the flowers are ready for pollination, the spadix heats up and emits a nauseating smell. This stench is so bad that the Indonesians call the plant ‘the corpse flower’.

Tuber: The inflorescence rises from a tuber, a swollen underground stem modified to store food for the plant. This tuber, more or less spherical in shape and weighing 70 kg or more, is the largest such structure known in the plant kingdom.

Leaf: After flowering, the inflorescence dies back and in its place a single leaf emerges. Reaching the size of a small tree, up to 7 m tall and 7 m across, the leaf consists of a sturdy glossy green stalk mottled with cream, which divides into three at its apex and bears numerous leaflets. Sugars made in the leaf are transported back to the tuber for storage as starch. Each year, the leaf withers before a new one develops, using the tuber’s energy stores. When the plant is ready to flower again, the tuber becomes dormant for up to four months before another inflorescence emerges, growing upwards at a rate of some 10 cm per day.

Pollination: Despite huge interest in titan arum, there has been no proper scientific study of its pollination to date. What is thought to occur is that when the flowers are ready for pollination the pollen-carrying insect enters, dives to the bottom of the inflorescence, deposits pollen on the stigmas and then stays there for 24 hours, emerging with the shedding of the pollen at around the same time the following day. Whether the insects are trapped or stay in the inflorescence because of some attraction remains to be verified, but the trap idea seems reasonable. The powerful foul smell and evening to night flowering suggests the pollinators may be beetles, or possibly flies which lay eggs in cadavers. The world expert on Amorphophallus , Wilbert Hetterscheid, has suggested it is likely that carrion beetles are the true pollinators.

Fruits: From the pollinated female flowers, the fruits develop inside the spathe chamber. Once they are ripe, the spathe withers completely exposing the bright scarlet fruits. These attract the attention of hornbills and other birds that eat them and disperse their seeds. The fruiting body looks rather like a giant version of the familiar lords and ladies ( Arum maculatum ) which appears in British hedgerows and woods in late summer.

While A. titanum undeniably has the bulkiest inflorescence in the Araceae family, it is not the largest in terms of vertical size. That honour belongs to Amorphophallus gigas which has a similarly sized spathe and spadix carried on a 3 to 4 m peduncle. Perhaps even more remarkable is the recently described A. pusillus from Vietnam, which has an inflorescence just 3 cm tall.

Threats and conservation

The rainforests of Sumatra are under massive threat of deforestation, as vast areas are logged for timber and to make way for oil palm plantations. It is estimated that Indonesia has now lost around 72% of its original rainforest cover, and the scale of deforestation is continuing at an alarming rate. As well as affecting titan arum numbers directly, the loss of habitat is also endangering species such as the rhinoceros hornbill ( Buceros rhinoceros ), which is an important seed distributor.

The Royal Botanic Gardens, Sydney and Bogor Botanic Gardens, Indonesia have been working together on conservation techniques for this rainforest giant. As well as investigating propagation techniques, surveys of wild plants have been undertaken and educational materials produced. This plant has previously proved very difficult to grow in cultivation. Ongoing research may provide the key to the continued survival of this spectacular member of the plant kingdom.

Uses

Titan arum is used for ornamental displays and can be an immensely popular visitor attraction.

Cultivation

This species has proved very difficult to cultivate, and there are only a handful of places in the world that do so. Even under optimum conditions the plant takes about six years to flower from seed.

At Kew, Amorphophallus titanum is cultivated in a tropical glasshouse, under conditions of high temperature and humidity, and kept in the shade. A well-drained, organic compost is used and plants are fertilised regularly during the growing season. Plants should be kept dry when dormant and watered when a new leaf/ inflorescence first appears. A.titanum is prone to rotting if over-watered. Plants should be re-potted whilst in the dormant stage (quite a feat when older corms can weigh over 70 kg). For pollination to occur, two plants at the right stages of maturity are required (one with receptive female flowers and one at the stage where male flowers are releasing pollen). The fruits are bright red when mature. Seed has been obtained from hand pollination of flowers by Kew staff on several occasions.

Titan arums at Kew

The first European botanist to encounter the species was the Italian Odoardo Beccari, who was travelling in tropical southeast Asia in 1878. He sent back seeds to his patron in Italy and one of the young plants that germinated from them was subsequently dispatched to Kew, where it flowered in 1889, exciting great public interest. In 1926, when it flowered again, the crowds attracted by the phenomenon were so large that the police were called to control them.

Kew now has several specimens of titan arum, which is one of the reasons we have had such an unprecedented number of flowerings in the last few years. You can almost always see a plant in leaf in the wet tropics zone of the Princess of Wales Conservatory.

Such is the unpredictable nature of the plant that we cannot tell whether it will be months, years or even decades before we next see a titan arum flower. However, we now have a large number of plants, and as such flowerings have become a remarkably common occurrence - we have seen more than three times as many titan arums flower at Kew in the last six years than in the previous 120 years!

Kew's recent flowerings

In 1996 a titan arum flowered at Kew for the first time for many decades. Six years later we saw an unprecedented three flowerings in as many months. This was the first evidence that Kew's horticulturists had finally cracked the secrets of cultivating this rare and unique plant.

1996 - A plant donated by Leiden University's botanic garden flowered at Kew and attracted vast crowds and intense media interest from around the world.

2002-3 - The specimens which flowered in 2002 and 2003 were grown both from seed donated to Kew in 1995 and from tiny micropropagated plants received from the botanic garden in Bonn in 1995. To build up the storage tubers, they were potted on into 750-litre containers in 1999 although they were in full leaf at the time. When the leaves died down, the largest tuber was placed in a 1,000-litre pot and the two smaller ones were put into new compost in 750-litre pots. For the next 14 months, they were grown behind the scenes at Kew in our Tropical Nursery. The plants were fed regularly with a high potash liquid fertiliser.

In January 2002, while the plants were dormant, they were potted up again. The largest of the three tubers, one of those grown from seed, weighed 75 kg (although since then we have had one, re-potted in winter 2004, which weighed an astonishing 91kg). As the new bud started to emerge the plant was transferred to the Princess of Wales Conservatory. During the day, the temperature is at least 24 ºC and at night it drops no lower than 19 ºC. The humidity is maintained at 70 to 80% – mimicking the conditions prevailing in the plant’s original rainforest habitat.

2005 to today - Thanks to an increase in the number of plants held at Kew, but equally the enhanced understanding and skill of Kew's horticulturists, we have seen several flowerings at Kew each year. We saw three flowerings in 2005, two more in 2006, two in 2007, one in 2008 and three in 2009. The third flowering of 2009 was Kew's biggest ever, measuring 2.48 m. The most recent flowering was in April 2016.

Odour investigations at Kew

Many of the 170 or so species of Amorphophallus produce a variety of obnoxious odours ranging from rotting meat, dung and rancid cheese to a nauseating gaseous stench. Their size does not always equate to their ability to generate a stink. Relative to A. titanum , the inflorescence of A. bulbifer is small, yet the gaseous stench it produces can make working in a glasshouse with it a sickening experience, as staff at Kew can testify. Some Amorphophallus species, however, produce pleasant odours; for example, A. haematospadix smells of bananas while A. dunnii has the odour of freshly chopped carrots.

Odours have been used to classify Amorphophallus species in the past, but the human nose can be deceived. When plants in the Princess of Wales Conservatory flowered in 1996, 2002 and 2003, scientist Dr Geoffrey Kite from Kew’s Jodrell Laboratory investigated the obnoxious smells that they produced. The strongest smells occurred on two consecutive evenings, firstly when the female flowers were ready for pollination and then when the male flowers were ready to shed their pollen.

The major components detected in the carrion and gaseous odours are the sulphur-containing compounds dimethyldisulphide and dimethyltrisulphide. The banana odour of Amorphophallus haematospadix appears to be due to isoamyl acetate while the carrot odour of A. dunnii consists almost entirely of 1-phenylethylacetate; the chocolate odour of A. manta has yet to be analysed.

One aim of the work is to compare the chemical nature of the odours with a modern classification of the genus based on DNA sequencing and a full scale morphological analysis. The chemical constituents of the odours might also provide clues to the pollinators of these plants since, for most species, the pollinators remain a mystery. The few observations available are for foul-smelling species and these seem to attract carrion beetles. This is in accordance with chemical data on the odour; dimethyldisulphide is known to attract carrion feeding or breeding insects and is used in a commercial lure for screw-worm flies.

Distribution

Indonesia

Ecology

Rainforest.

Conservation

Classified as Vulnerable (V) in the 1997 IUCN Red List of Threatened Plants.